code: def backward(self):
      self.loss.backward()
  
file_path: torch-points3d/test/mock_models.py
imports: from torch_points3d.models.base_model import BaseModel
  from torch import nn
label_desc: Use image node_id 4 for calling the DifferentiableMockModel obj's underlying member method code with example usage: obj.backward() without return types